The establishment of links provides a benefit in grouping adjoining carriageway sections that have a similar traffic volume and characteristics (e.g. mix, traffic group) such that they can be considered to have a common estimate. So, for example, a network of 1500 carriageway sections may only have 1000 links. This benefits the count strategy in the following manner. Say a count programme that over a five-year period covers 500 count sites. For a network of 1000 links, this equates to a sample rate of 50% of the contiguous traffic sections. If there are 1500 links, this equates to only a third. Thus the sample rate is much higher and better informs the traffic patterns on the network.

Generating Links

As described previously, if the asset manager is generating (or regenerating links), then this is the step whereby that is completed. A recommended process is as follows:

• The generation of links starts at the first section of a road being assigned a link identification number (link id).
• This same link id is assigned to the next carriageway section on the road if it varies by less than the maximum of 10% or 50 vpd and is in the same traffic group and ONRC/ONF framework. If the variation exceeds this, a new link id is assigned and the process continues until the end of the road is reached or the variation trigger is exceeded.
• If the variation trigger is exceeded or the next section is a different traffic group or ONRC/ONF category, the process restarts with a new link ID
• This process continues until the end of the road is reached, and so on, until the end of the road is reached. The process then moves onto the next road with a new Link ID.

There needs to be splits in links where the road section changes by traffic group or urban/rural split for example as there signify a change in the characteristic of the traffic flow.

Links are assumed to be along the same road and to be contiguous.

In the example procedure above, the sections in the link have an estimate that is within a certain range of a single estimate as a representative value. There are more complex arrangements one can use. One example is to define contiguous sections if they can be linked by a linear change in traffic volume i.e. an approximate straight line, sloping up or down or flat. The link start is then reset if the next AADT varies too much from the value predicted by the slope regression.

Where links contain multiple count sites, consideration should be given to whether the multiple sites are required resulting in splitting the link. Alternatively, keeping the most appropriate count site and reassigning the budget from the other sites to other sites or achieving some cost savings.

If links are not being (re)generated, they should be checked for any changes after the annual review process. Checks should include:

• Link splits may need to be revised where they have multiple count sites
• Links splits maybe necessary where there are multiple ONRC/ONF categories

Reviewing links along key routes with frequent count sites that the estimates and link splits appear sensible.